Disk drives are information storage devices that include at least one rotatable disk with concentric data tracks containing the information, and one or more magnetic heads (or transducers) for reading data from and writing data to the data tracks. The data read by a magnetic head are further amplified by a pre-amplifying circuit before they are used by other parts of the disk drive system. In advanced disk drives, magneto-resistive (MR) heads are used for reading and writing data. An MR head generally includes an MR read element or sensor for reading data from the data tracks and an inductive write coil for writing data to the data tracks.
In order to guide the MR head over a data track while the disk is rotating, servo sectors are placed in the track along with data sectors. These servo sectors are read by the same MR sensor that reads the data sectors. The presence of the servo sectors makes it necessary to interrupt the writing during write cycles of the MR head as the head passes over a servo sector to allow the head to read the sector. Furthermore, with the disk rotating at a very high speed, it is crucial that the MR head be ready for reading data almost immediately after it is switched from write or idle mode to read mode, to avoid skipping portions of the track. Typically, it is desirable that the delay be less than 1 microsecond. Such a short duration, however, is difficult to achieve in existing prior art pre-amplifying circuits where an MR head is used. The thermal response of the MR sensor, due to the variation in the MR sensor bias current as the MR head mode changes, prevents the MR sensor from reaching steady-state until several microseconds after the MR head changes its operational state.
Pre-amplifying circuits for MR heads are described, for example, in U.S. Pat. Nos. 5,122,915 and 5,032,935. Due to the thermal response problem described above, the prior art pre-amplifying circuits typically generate undesirable transient voltages at their outputs as the MR head switches from write or idle mode to read mode. These transients result because the MR sensor is biased with a different current in each operational state.
Normally, the MR sensor is not biased when the MR head is in write mode to avoid overdriving the amplifying circuit, or in idle mode to save power. Such overdriving is caused mainly by the capacitance coupling between the MR sensor and the write coil of the MR head. When the MR head is in write or idle mode, the amplifier coupled to the MR sensor is usually switched off. Upon the MR head switching to read mode, the MR sensor is again biased with a DC current. This bias current causes the temperature of the MR sensor to rise from its value when the MR head was in write or idle mode until the steady-state temperature is achieved. The temperature rise results in a change in the MR sensor resistance, which depends on the temperature coefficient of the MR sensor. The variation in the MR sensor resistance in turn produces a transient voltage at the output of the read-back pre-amplifying circuit, thus preventing the MR head from properly reading data for several microseconds immediately after the MR head switches to read state.
Therefore, there remains a need for a MR head pre-amplifying circuit that does not generate voltage transients at its output as the MR head switches among its operational modes.